With bicycles having a multi-sprocket device for a front or rear gear, the speed of the bicycle is changed by manipulating a shift lever assembly mounted on a suitable portion of the frame of the bicycle to operate a rear derailleur or front derailleur for engaging the chain with a selected sprocket of the multi-sprocket device.
The shift lever assembly comprises a pivot fixed to the bicycle frame, and a shift lever movably supported by the pivot. When pivotally moved, the shift lever pulls or pays out a wire connected to the derailleur. The longitudinal movement thus given to the wire is transmitted to the derailleur to operate the derailleur.
The shift lever is usually attached to the downtube of the bicycle frame, but this involves a hazard since the lever needs to be manipulated for a speed change by the hand removed from the handle grip. Accordingly, shift lever assemblies have been proposed which are adapted to be manipulated with the hand grasping the handle grip.
For example, Examined Japanese Patent Publication SHO 48-23627 discloses a shift lever assembly comprising a wire winding portion and a shift lever which are separate members but are rotatable on the same pivot. A gear disposed on the wire winding portion and a pawl disposed on the shift lever and biased toward the outer periphery of the gear provide a one-way clutch and also a stepwise rotational advancing mechanism, the shift lever being so biased as to return to a specified neutral position.
When the shift lever in the neutral position is rotated through a predetermined angle, the rotation is transmitted to the wire winding portion through the clutch mechanism. When released from the hand, the shift lever automatically returns to the neutral position, with the wire winding portion held in the rotated position owing to friction imparted to this portion. The shift lever, when rotated again through the predetermined angle, further rotates the wire winding portion through a specified angle in the same manner as above. In this way, the shift lever which returns to the neutral position at all times is repeatedly rotated by the predetermined amount, whereby the wire winding portion is rotatingly advanced stepwise by the desired amount to operate the derailleur. Furthermore, the shift lever assembly is so adapted that the wire winding portion can be rotated stepwise by repeatedly rotating the shift lever, biased to return to the neutral position, through the predetermined angle not only in the forward direction (e.g., wire winding direction) but also in the reverse direction (wire paying-out direction).
Nevertheless, the disclosed shift lever assembly still has the following problems.
First, the one-way clutch mechanism comprises a gear which needs to be smaller in diameter and therefore in the number of teeth if the mechanism is to be compacted, and a pawl which must be in engagement with the tooth of the gear whenever the shift lever is in its neutral position, so that the angle through which the shift lever is moved at a time needs to be restricted to an angle corresponding to the circumferential pitch between the teeth of the gear. The restricted angle of rotation of the shift lever then will not always correspond to the position to which the derailleur is to be guided, with the result that a satisfactory speed change will not be made. For the derailleur to shiftingly engage the chain with a selected sprocket of the multi-sprocket device, it is required that the chain guide of the derailleur be brought to a position exactly corresponding to the selected sprocket, whereas this can not always be achieved by the shift lever.
Second, the amount of advance afforded by the rotation of the shift lever through the predetermined angle at a time is accumulated stepwise to obtain the desired amount of rotation of the wire winding portion, so that even if it is attempted to shift the chain, for example, from a diametrically largest sprocket (low-speed sprocket) of the rear gear to a diametrically smallest sprocket (high-speed sprocket) quickly, the rider must follow the cumbersome procedure of repeatedly moving the shift lever, failing to effect a rapid speed change in accordance with the situation encountered.